The induction of plasmacytomas (PCTs) in the peritoneal cavity of BALB/cAn mice by plastics, paraffin oils or silicone gels is a model system for identifying the steps and stages of neoplastic development. The initial oncogenic event in this process is a chromosomal translocation that illegitimately recombines c-myc with an Ig locus gene resulting in deregulation of c-myc transcription. The focus of the work in the last year has been on determining when the initiating oncogenic mutation (a chromosomal translocation that deregulates c-myc transcription) takes place. Recent evidence from a highly sensitive nested PCR detection method indicates this step probably takes place before the introduction of the peritoneal inducing agent. Peritoneal PCT formation is dependent upon a susceptible genotype. Previous work has shown that genes on chromosome 4 contribute to this susceptibility by acting during the progression phase of the process. We seek to find the mechanism of action of these genes. Using PCR we now can ask whether there is also a genetic basis for developing the illegitimate recombinations of c-myc and Ig switch region sequence, or for influencing the number of these translocation events in normal B cell development. Continuing studies on the pathogenesis of PCT development initiated by different inducing agents have shown that silicone gels are highly efficient inducers of the microenvironment that permits progression. We are attempting to identify the active components and properties of the silicone gel that are responsible for PCT progression.